Gas turbine engine superalloy turbine blades and/or turbine vanes are coated in the airfoil region and sometimes in the platform region and even the shank of the root region with a simple or Pt-modified diffusion aluminide coating to provide a bond coat for thermal barrier ceramic coating, protection against deterioration by high temperature oxidation, or mild salt promoted corrosion processes that occur at the operating temperature experienced during use. Formation of the diffusion aluminide coating is accompanied by dimensional growth which can be tolerated in the those regions of the turbine blade/vane.
However, the fir tree region or other attachment region of the superalloy turbine blade or vane cannot tolerate such dimensional growth since it may exceed the dimensional tolerance of fitting/mating surfaces leading to assembly problems and possible mechanical failure in highly stressed attachment regions, e.g. fir tree roots. Chromizing of the fir tree region or other attachment region portion concurrently with aluminizing of the other regions of the turbine blade/vane has been attempted to protect the fir tree region or other attachment region from lower temperature corrosion without experiencing unwanted dimensional growth there. In one known method, a first mask comprising chromising composition is arranged on the selected region of the superalloy turbine component and an aluminising mask is arranged on the chromising composition.
The chromising composition comprises chromium powder, ferrochrome powder or other chromium containing powder, an inert refractory diluent powder, and a halide activator mixed with binder to form a slurry that is applied to the region to be coated. The first mask is covered by a second mask comprising an aluminizing mask, which can be a slurry coating or alternatively a particle-filled making box. The second mask comprises nickel powder, nickel oxide powder or nickel alloy powder, refractory powder such as alumina, and an inorganic resin binder.